1. Field of Invention
The present invention relates to an optical projection system. More particularly, the present invention relates to using a wire grid polarizing device, so that the light can be mixed by a transmission manner.
2. Description of Related Art
The optical projection system has been very common in our daily life. For example, the small image content can be projected to a large screen for view by the user. Particularly, when the other technologies have also been well developed, such as the development of the liquid crystal displaying technology, they can be implemented together with the optical projection system, so as to achieve the function to display the image. For example, liquid crystal projection TV is one of products by modem technologies.
The conventional optical projection system, which is also implemented with the liquid crystal (LC) displaying device, is designed as shown in FIG. 1. The conventional optical projection system in FIG. 1 is designed by using the property of polarization beam splitter (PBS). A light beam 100 first enters a color splitter 150, which splits the light beam 100 into, for example, a blue beam 104 and a color mixing light 102. This color mixing light is composed of green light and red light. The blue beam 104 travels along a light path, and the color mixing light 102 travels along another light path. The light path for the blue beam 104 is first described. The blue beam 104 then is led by the reflection mirrors 152 and 154, so as to become the blue beam 106 that travels toward a PBS 158b through a field lens 156b. The PBS 158b reflects a light component with a specific polarization state onto a liquid crystal on silicon (LCOS) panel 160b. The LCOS panel has multiple pixels, usually being arranged by an array structure. Each pixel of the LCOS panel can be controlled to change the polarization state of the incident light and then the incident light is reflected, wherein the changed polarization state is associating with the PBS. As a result, the effect of color brightness or gray level can be adjusted.
On the other hand, the color mixing light 102 enters a color splitter again to be split into a red beam 108 and a green beam 110. When the red beam 108 and the green beam 110 are split, the light paths are similar to the blue beam 104, and respectively enter the PBS 158r and 158g as well as the LCOS panels 160r and 160g, so as to form the image patterns with respect to the primary colors.
Then, the three beams, which are reflected by the LCOS panels 160r, 160g, and 160b, enter a color-combination prism 162, so as to combine the three beam into a color mixing light. The color mixing light then propagates out from another face of the color-combination prism 162 and reaches to the projection lens set 164. Wherein, the color-combination prism 162 usually includes an X-cube.
With respect to the conventional design in FIG. 1, the design mainly uses the PBS and has several disadvantages. The operation mechanism of the PBS is shown in FIG. 2. A non-polarized light beam enters a slant surface 166 of the PBS. It can allow the light with a specific polarization state to pass but reflect the light with another polarization state. However, for the actual light, the light beam is not a spot beam and is incident by a two-dimensional plane. FIGS. 3A–3C show the situation for the actual beam entering the PBS. In FIG. 3A, when the light is incident by a 2-dimensional plane, at the cross-sectional plane as shown in the drawing, the behaviors can be like the function in FIG. 2. However, the other cross-sectional plane (not shown) then cannot be completely split into two light beams with different polarization states. In FIG. 3B, for the reflected light beam in 2-dimensional circular area, the horizontal axis represents the direction in FIG. 2. It has pure polarization state, represent by, i.e., circle points. However, the position deviating from the horizontal axis will be mixed with a little portion of light with the other polarization state, indicated by vertical double-arrow. Thereby, when a complete dark state or dark field is intended, a little light leakage occurs. In FIG. 3C, the light leakage usually occurs at the regions 180. For the current requirement, the brightness contrast is better than 1000:1. This light leakage would affect the product quality.
In order to solve the forgoing issue of PBS, the interfacing surface 166 is necessary to use the glass with low birefringence to maintain the good polarizing property. Moreover, in order to reduce the light leakage, it is necessary to be coated with a ¼-wave compensation layer to reduce the light leakage. In this situation, the fabrication cost is high and the system is not easy to be maintained. It even needs a grading coating process, which process is difficult to be performed.
Conventionally, an alternative design is using a wire grid polarizer (WGP) to replace the PBS. FIG. 4 is drawing, schematically illustrating a conventional optical projection system using WGP. In FIG. 4, a light source 300 emits a white light, which is split by a color splitter 302 into a blue beam 304 and a color mixing light 306. The color mixing light 306 as a yellow beam is first described. The color mixing light 306 travels through the condenser lens 308, and then is reflected by the reflection mirror 314 to change the travelling direction. The light beam is then enters another color splitter 316 and split into a red beam 318 and a green beam. With respect to the red beam 318, it passes through a WGP 320, wherein a light component with a polarization state would pass and reach to a liquid crystal reflection panel 322. The liquid crystal reflection panel 322, by a polarization state, reflects the incident beam onto a WGP 320. At this moment, the reflected light beam carries an image pattern. The WGP 320 then reflects the light beam onto a color-combination prism 162. For this conventional design, the liquid crystal reflection panel 322 is perpendicular disposed, with respect to the corresponding incident surface of the color-combination prism 162. The light beam enters the color-combination prism 162 by the reflection from the WGP 320. The other blue beam (B) 304 and the green beam (G) splitting from the color mixing light 316 are similarly designed. For the blue beam (B) 304, it can further use the condenser lens 310 and the relay lens 312.
The invention has discovered some issues in this conventional design in FIG. 4, such as the deformed WGP, which would cause the chromatic aberration and so on. With respect to the discovered issues, the invention has proposed a novel design as to be described later.
In addition, the other conventional optical projection systems are shown in FIG. 9 and FIG. 10. In FIG. 9, basically, this conventional design uses a condenser lens 804 to condense the light beam from the light source and then the condensed beam afterward enters the color splitter 806. The subsequent light path is implemented with the LC panel 812. For this design, since the condenser lens 804 is designed with a single-side telecentricity or non-telecentricity, the light cone angle is quite large. Then, the subsequent color splitter 806 needs a gradient coating for reducing the color non-uniformity caused by cone angle. The red beam 802R and the green beam 802G are again split by the color splitter 818. The green beam 802G is reflected by the reflection mirror 822. Since the length of the light paths is different, a pair of relay lenses is needed to adjust the phase difference. The design on light path can be referred to FIG. 9, and should be understood by the skilled artisans. The further description is omitted.
Further still, FIG. 10 is a design using the WGP 916 with the liquid crystal reflection panel 918. However, it has the same problems as in FIG. 9. That is, the light beam form the light source 900 is first condensed by the condenser lens 902 and then reaches to the color splitter 904. Since the condenser lens 902 is not designed by telecentricity, a color splitter needs the gradient coating. Furthermore, since it is not a telecentric design, the light path of each color beam is restricted by the color splitter and cannot be independently adjusted. This also causes the design difficulty to get the light paths in equal length.